At the present time acrolein is industrially produced by vapour phase catalytic oxidation of propylene. All the attempts to improve this process have hitherto been concerned with the development of catalysts which give the highest possible conversion of propylene and the highest possible selectivity for the desired acrolein.
Thus, French Patent No. 2 093 773 describes the manufacture of acrolein by vapour phase catalytic oxidation of propylene with molecular oxygen in the presence of an oxide catalyst in which the composition of the catalytic elements, expressed as atomic ratio, is the following:
Co.sub.2.0-20.0 Fe.sub.0.1-10.0 Bi.sub.0.1-10.0 W.sub.0.5-10.0 Mo.sub.2.0-11.5 Si.sub.0.5-15.0 Z.sub.0.005-1.0 with W+Mo=12.0 and Z denoting an alkali metal. This catalyst can be prepared by mixing aqueous solutions of ammonium molybdate and of ammonium paratungstate, adding solutions of cobalt nitrate, of iron nitrate and of bismuth nitrate to the aqueous mixture and then adding an aqueous solution of an alkali metal hydroxide or carbonate and then colloidal silica as a source of silicon, by moulding the substance obtained and by calcining it at 350-600.degree. C. in a stream of air.
U.S. Pat. No. 3,855,308 describes the preparation of acrolein by vapour phase catalytic oxidation of propylene with molecular oxygen in the presence of an oxide catalyst in which the composition of the catalytic elements, expressed as atomic ratio, is the following:
Co.sub.2.0-20.0 Fe.sub.0.1-10.0 Bi.sub.0.1-10.0 W.sub.0.5-10.0 Mo.sub.2.0-11.5 Si.sub.0.5-15.0 Tl.sub.0.005-3.0 Z.sub.0-3.0 with W+Mo=12.0 and Z denoting an alkali metal or alkaline-earth metal. The raw materials employed for forming the catalyst may be the oxides of the various metals, but also, depending on circumstances, the nitrates, carbonates or hydroxides. In the case of Mo and W, the salts of acids, such as ammonium molybdate and ammonium tungstate, are recommended. Thus, according to this US patent a catalyst is prepared by mixing aqueous solutions of ammonium molybdate and of ammonium paratungstate respectively, by adding solutions of cobalt nitrate, of iron nitrate and of bismuth nitrate respectively, and then an aqueous solution of alkali metal hydroxide or carbonate and then colloidal silica as a source of silicon, by concentrating the system by evaporation, by adding a support if necessary and by following with an evaporation, a mixing of the resulting substance and a calcination at 350-600.degree. C.
Japanese Patent Showa 45-125 359 describes a vapour phase process for the manufacture of acrolein by catalytic oxidation of propylene with air or oxygen in the presence of a catalyst of formula: EQU Ni.sub.a Co.sub.b Fe.sub.c Bi.sub.d Me.sub.e H.sub.h Mo.sub.f O.sub.g
in which:
a=0-20, b=0-20 with a+b between 0.5 and 20, c=0.5-8, d=0.1-7, 0&lt;e.ltoreq.2, h=0-0.3, f=12, g=36-90; PA1 Me is one out of Sn, Zn, W, Cr, Mn and Ti; and PA1 H is at least one out of K, Rb and Cs. PA1 the limitation of the overoxidation of the products formed, which takes place in the presence of molecular oxygen; according to the present invention, since the operation is carried out in the absence of molecular oxygen, the formation of CO.sub.x (carbon monoxide and carbon dioxide), degradation products, is reduced, and this allows the selectivity for acrolein to be increased, as shown hereinafter by the Comparative Examples 4, 8, 12 and 16; PA1 the selectivity for acrolein remains good when the degree of reduction of the solid composition increases; PA1 once it has undergone a reduction and a gradual loss of its activity, the solid composition can be easily regenerated by heating in the presence of oxygen or of an oxygen-containing gas after a certain period of use; after the regeneration the solid recovers its initial activity and can be employed in a new reaction cycle; PA1 the separation of the stages of reduction of the solid composition and of its regeneration makes it possible: PA1 a is between 0 and 5, limits included, PA1 b is between 0.5 and 5, limits included, PA1 c is between 0.1 and 10, limits included, PA1 d is between 0.5 and 10, limits included, PA1 e is between 0 and 10, limits included, PA1 f is between 0 and 15, limits included, PA1 g is between 0 and 1, limits included, PA1 h is between 0 and 2, limits included, and PA1 x is the quantity of oxygen bonded to the other elements and depends on their oxidation states, in the manufacture of acrolein by propylene oxidation, the said solid composition reacting with the propylene according to the redox reaction (1): EQU solid.sub.oxidized +propylene.fwdarw.solid.sub.reduced +acrolein(1). PA1 in the case of molybdenum, ammonium molybdate and molybdic acid and, in the case of tungsten, ammonium tungstate and tungstic acid, PA1 in the case of cobalt, bismuth, nickel and iron, the nitrates, carbonates and hydroxides, such as cobalt nitrate, bismuth nitrate, nickel nitrate and ferric nitrate, PA1 in the case of tin, tin chloride and tin hydroxide, and PA1 in the case of potassium, potassium hydroxide, carbonate or nitrate, and, in general, any compounds capable of forming an oxide by calcination, namely metal salts of organic acids, metal salts of inorganic acids, metal complex compounds, organometallic compounds and the like.
To prepare this catalyst, aqueous solutions of Ni, Co, Fe, K (and/or Rb, Cs), Bi and Me compounds may be added to an aqueous solution of a molybdenum compound (ammonium molybdate, molybdic acid or molybdenum oxide), then a support such as alumina, silicon carbide and silica (silica sol or silica gel) may be added and then the resulting mixture is heated to dryness, is calcined at approximately 500.degree. C. and is converted into pastilles.